Arc detection and handling in radio frequency power applications

ABSTRACT

A radio frequency power delivery system comprises an RF power generator, arc detection circuitry, and control logic responsive to the arc detection circuitry. A dynamic boundary is computed about the measured value of a parameter representative of or related to the power transferred from the power generator to a load. A subsequently measured value of the parameter that exceeds the computed dynamic boundary of the parameter indicates detection of an arc. Upon detection of an arc, power delivery from the generator is interrupted or adjusted, or other action is taken, until the arc is extinguished. By employing dynamic computation of arc detection boundaries, the invention allows for arc handling in RF power deliver systems regardless of whether the system has reached a stable power delivery condition.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to radio frequency power delivery, andmore particularly to the detection and avoidance of arcing in radiofrequency powered plasma processes.

2. Brief Description of the Prior Art

Radio frequency (RF) powered plasma processes are commonly used in themanufacture of semiconductors, flat panel displays, data storagedevices, and in other industrial applications. While RF power suppliesare typically well protected against sudden changes in load impedance,they generally are not designed to detect and respond to changes inplasma impedance caused by arcing within a process chamber. As a result,an RF power supply may continue to feed energy into incipient arcs thatdevelop within a plasma process, which in turn may cause serious damageto the surface of a workpiece or even to the processing equipmentitself.

In DC powered plasma processes, the problem of arcing has long beenstudied, particularly in reactive sputtering applications. In a reactivesputtering process, arcing often results from charge buildup andeventual electrical breakdown on the surface of dielectric filmsdeposited on the sputtering target or chamber walls. Problems of arcingin DC plasma processes have been addressed by through the use ofsophisticated arc handling systems capable of detecting arcs and ofemploying any number of techniques to mitigate their severity, such astemporarily interrupting power or reversing the polarity of outputvoltage. In critical applications, the time during which the outputvoltage is removed is taken into account to adjust processing time sothat the total energy delivered to the plasma is controlled and limited.In DC systems, it has also long been recognized that pulsing the DCoutput or reversing the output polarity at a certain repetition rate andduty cycle can reduce the tendency of arcs to develop.

RF power has been seen as an alternative technology that may be used tosputter an insulator directly while avoiding altogether the arcingproblems in DC sputtering processes. Only recently has it beenrecognized, however, that occasional arcing occurs in RF processes aswell, and that for sensitive film properties or geometries this RFarcing can be equally as damaging. Arcing in RF powered systems mayresult from charge buildup across gate-electrode patterns onsemiconductor wafers or upon polymer coatings on chamber surfaces. Otherfactors include defects in the reactor or chamber hardware, degradationof the protective chamber anodization layer, differences between theelectrical potential across tool parts, or even simply the magnitude ofthe RF power being applied. In any event, handling and avoidance ofarcing requires the capability of both rapidly detecting the onset of anarc and rapidly interrupting or removing the output power so as toreduce the energy delivered into the arc.

In one approach, arc detection and avoidance in RF systems has beenattempted based upon establishing a predetermined threshold of a powerdelivery parameter, such as reflected power. The occurrence of an arc isinferred from a sudden rise or spike in reflected power that exceeds thepredetermined threshold. This approach is not effective, however, whilethe power transfer of the system is being tuned, i.e., before thereflected power of the system has been brought to a steady state valuethat is below the predetermined threshold. The threshold approach isalso limited in that arcing in an RF processing application does notalways lead to an increase in reflected power. Depending on the state ofthe match network, an arc may in fact reduce the reflected power, andtherefore not trigger an arc detection in a simple threshold circuit.

Another approach to RF arc detection correlates the derivative, or timerate-of-change, of a power delivery parameter to an arcing condition.Some RF arcs may develop slowly, however, over a period of 1 microsecondor longer, and may therefore go undetected by a derivative detector.Moreover, the derivative detector has increasing gain with frequency upto a point where practical limitations restrict the bandwidth. As aresult, the derivative detector becomes more sensitive to noise athigher frequencies of operation.

SUMMARY OF THE INVENTION

This invention provides methods and systems for detection and reductionof arcing in RF power delivery applications. In one aspect of theinvention, an RF power generator applies power to a load, such as aplasma in a plasma processing system. A dynamic boundary is computedabout the measured value of a parameter representative of or related tothe power transferred from the power generator to the load. Asubsequently measured value of the parameter that exceeds the computeddynamic boundary of the parameter indicates detection of an arc. Upondetection of an arc, power delivery from the generator is interrupted oradjusted, or other action is taken, until the arc is extinguished.

In one embodiment of the invention, a plasma processing system comprisesan RF power generator that furnishes power through an impedance matchingnetwork to a plasma load. Instantaneous values of reflected powerbetween the generator and load are measured, while the match network istuning as well as during fully tuned, steady state power delivery. Aboundary comprising upper and lower values of reflected power about themeasured value is computed dynamically and evaluated by a controllercircuit. If a subsequently measured value of reflected power exceedseither the upper or lower boundary limit, the occurrence of an arc isindicated. The controller circuit interrupts power from the generator tothe load for a brief interval to quench the arc. If the reflected powerhas fallen back within the boundary limits, normal power delivery isresumed.

In other embodiments of the invention, any one of a number of availablepower delivery parameters or signals is used alone or in combination fordetection of arcs in RF powered plasma systems. In addition to reflectedpower, dynamic boundaries of the invention are computed based uponmeasurements of parameters including, but not limited to, loadimpedance; voltage, current and phase; forward power, delivered power,VSWR or reflection coefficient; magnitude level variations in theharmonic output; DC bias developed on a process workpiece or target;changes in the RF spectrum harmonics or acoustic interferences; orvariations in ion saturation currents, electron collision rates, orelectron densities within the plasma.

In another embodiment of the invention, an RF power delivery systememploys parallel arc detection circuitry. A slow-filtered measurement ofa power delivery parameter is used in to compute dynamic arc detectionboundaries, in conjunction with one or more user-selected constants thatdetermine sensitivity of the detection circuitry. A fast-filtered valueof the power delivery parameter is compared to the computed detectionboundaries in order to detect occurrence of an arcing condition. In thisway, a flat pass band is created between the cut-off of the slow filterand the cut-off of the fast filter. As a result, optimal sensitivity canbe maintained over a range of input frequencies as compared for exampleto arc detection based upon derivative techniques.

By employing dynamic computation of arc detection boundaries, theinvention allows for arc handling in RF power deliver systems regardlessof whether the system has reached a stable power delivery condition.Continuous monitoring and handling of arcing events in RF applicationsallows for improved process quality and throughput with better yields.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a plasma processing system in accordance with oneembodiment of the invention.

FIG. 2 illustrates a process and circuitry for arc detection andhandling in an RF power delivery application in accordance with oneembodiment of the invention.

FIGS. 3 a and 3 b illustrate detection and handling of arcing in an RFpower delivery application in accordance with one embodiment of theinvention.

DETAILED DESCRIPTION

FIG. 1 illustrates a plasma processing system in accordance with oneembodiment of the invention. Processing system 10 comprises RF powergenerator 12 that delivers RF power through impedance matching network14 to a plasma 16 within plasma chamber 18. Instantaneous values offorward power P_(F) and reflected power P_(R) are measured at the outputof generator 12 and communicated to control logic 20, which controls adisconnection circuit at the output of power generator 12.

FIG. 2 illustrates a process and circuitry for arc detection andhandling in an RF power delivery application in accordance with oneembodiment of the invention. Measurements of forward power P_(F) andreflected power P_(R) are filtered through filters 102, 104, and 106.Absolute offset values O₁ and O₂, and multipliers k₁ and k₂, areuser-selected inputs that determine the sensitivity of the arc detectioncircuitry. The sum of offset O₁ from slow-filtered reflected power andmultiplier k₁ applied to filtered forward power sets the upper reflectedpower limit of dynamic boundary 120, while the sum of offset O₂ andmultiplier k₂ times filtered forward power, inverted through inverter108, sets the lower reflected power limit of the dynamic boundary. Upperand lower limits of dynamic boundary 120 are continually recomputed anddynamically updated in response to changes in P_(F) and P_(R).Comparators 110 and 112 compare the difference between the fast-filteredvalue of reflected power P_(R) to upper and lower limits, respectively,of the dynamic boundary. Control logic 114, which is responsive to thecomparisons generated by comparators 110 and 112, controls disconnectionswitch 116 of an RF power generator.

In the embodiment of FIG. 2, a fast-filtered value of reflected powerP_(R) that falls outside the upper and lower limits of the dynamicboundary 120 indicates detection of an arcing condition in the processor application. FIG. 3 a illustrates examples of arcing conditions 206and 208 that cause reflected power P_(R) to exceed dynamic boundary 202,204. Referring again to FIG. 2, in response to the arc detection signalreported from either of comparators 110 or 112, control logic 114interrupts power delivery from the RF power generator by openingdisconnection switch 116. Power delivery is interrupted for a timesufficient to quench the arc, at which time control logic 114 instructsdisconnection switch 116 to close and normal power delivery resumes.

Dynamic boundary limits are set so as to maximize arc detectionsensitivity while minimizing the occurrence of false positivedetections. In an representative RF plasma processing application, forexample, requiring RF power delivery in the kilowatt range, reflectedpower offsets of 50-100 watts and forward power multipliers of 4%provided acceptable arc detection performance. The filtering timeconstants applied to measurements of power delivery parameters, such asforward and reflected power, are similarly chosen based upon performancetradeoffs. Thus, for example, even though some arcs may take amillisecond to develop, they still develop much faster than the expectednatural change in impedance presented to the generator due to the tuningactions of an impedance matching network. The slow filter can thereforebe set up to have a time constant of one or two ms and still follownormal changes in plasma characteristics. The time constant of the fastfilter is typically chosen based on noise considerations, but isgenerally at least 10 times longer than that of the slow filter. Thus,even though the time constant of the slow filter may be on the order of1 ms, arcs can generally be detected in a fraction of the time constantof the fast filter.

FIG. 3 a illustrates the further ability of the invention to detect andrespond to arcing conditions during tuning or other non-steady statepower delivery conditions. To have arc detection while a match networkis still tuning, or in systems that never achieve perfect tuning such asfixed match systems with variable frequency RF generators, embodimentsof the invention utilize dynamic limits set about the nominal value ofthe signal being monitored. When power is initially applied from an RFgenerator to a plasma load, for example, an impedance mismatch istypically present between the load impedance and the output impedance ofthe generator. As a result, reflected power is initially high. Animpedance matching network operates to tune the system to improve powertransfer by reducing reflected power, as illustrated for example by thedecreasing reflected power curve 200 of FIG. 3. Upper 202 and lower 204limits of a dynamic arc detection boundary are computed continuously andtrack the instantaneous level of reflected power. As a result, arcingconditions 206 and 208 may be detected and handled during power tuningwithout waiting for the power delivery to reach a steady statecondition. Moreover, arc detection and handling may continue to operatein the event that load conditions change and retuning of the systemoccurs.

Once arcs are detected, many options are available for handling andextinguishing the arcs. Power delivery may be interrupted, for example,or simply reduced. In one embodiment of the invention, power delivery isinterrupted upon initial detection of an arc for a period of 50 to 100μs, a value that permits a typical processing plasma to return to itsnormal (i.e. non-arcing) state. In the event the arc is not quenched, afurther interruption is triggered for a longer time, e.g. double thelength of the first interruption period. This increase in time iscontinued until either the arc is quenched or a pre-determined number ofattempts to quench the arc has failed, in which case the generator shutsdown to protect the system. It has been found that RF power delivery maybe interrupted in such typical applications for as long as 10milliseconds with the impedance of the plasma returning quickly (withinapproximately 20 μs) to the value present before interruption.

In a further aspect of the invention, a sample-and-hold feature isprovided in arc detection circuitry in order to address occurrences ofpersistent or “hard” arcs. Referring to FIG. 2, in one embodiment of theinvention, control logic 114 is equipped to deliver a Hold signal toslow filter 104 upon detection of an arcing event. The Hold signalcauses the output of slow filter 104 to be maintained at the valueexisting immediately prior to occurrence of the arc. As illustrated inFIG. 3 b, the fast-filtered value of reflected power is compared toconstant upper and lower arc detection boundaries based upon the nominalvalue maintained by the slow filter, in order to determine whetherconditions of the system have returned to the state prior to occurrenceof the arc.

The invention has been described with reference to power deliverysystems for plasma processing applications that furnish power in thekilowatt range at radio frequencies, for example 13.56 MHz. The arcdetection and handling techniques of the invention may be employed,however, in any apparatus, application or process that furnishes powerat any alternating current frequency. Arc detection and handlingcircuitry of the invention may be implemented within a power generatoror match network, in whole or in part, or may alternatively be providedand/or operated separately from other system components. Although theinvention provides for arc detection and handling during tuning of apower delivery system, or in other conditions where perfect tuning isnever achieved, the invention does not require presence or use of animpedance matching network.

The power delivery parameters upon which dynamic arc detectionboundaries are computed are chosen to ensure that arcs are detectedreliably with an acceptable false detection rate. Secondaryconsiderations include cost, ease of use, and the ability to classify,count and report arc events. While embodiments of the invention havebeen described in which dynamic arc detection boundaries are computedbased upon measurements of forward and reflected power, otherembodiments of the invention compute dynamic boundaries based upon otherpower delivery parameters such as load impedance; voltage, current andphase; VSWR or reflection coefficient; magnitude level variations in theharmonic output; changes in the RF spectrum harmonics or acousticinterferences; or even variations in electron collision rate or electrondensity.

In one embodiment of the invention, dynamic arc detection boundaries arecomputed based upon a DC bias that develops on a process workpiece ortarget. In addition to being fast and reliable, the approach isadvantageous in that the continued presence of the DC bias after thepower delivery has been interrupted gives a direct indication that anarc has not yet been extinguished. In cases where a natural DC bias isnot developed, a DC power supply is used to inject a DC bias for thepurpose of detecting arcs. One potential complication is that the biasdetection must be done on the chamber side of the match (that is, thedetection would be incorporated in the match), while the arc detectionsignal must be provided to the RF generator.

Although specific structure and details of operation are illustrated anddescribed herein, it is to be understood that these descriptions areexemplary and that alternative embodiments and equivalents may bereadily made by those skilled in the art without departing from thespirit and the scope of this invention. Accordingly, the invention isintended to embrace all such alternatives and equivalents that fallwithin the spirit and scope of the appended claims.

1. A method of delivering radio frequency power to a load, comprising:a) providing an RF power generator that transfers radio frequency powerto a load; b) measuring a value of a parameter related to the powertransfer from the RF power generator to the load; c) computing a dynamicboundary about the value of the parameter; d) measuring a subsequentvalue of the parameter; and e) indicating the occurrence of an arc ifthe subsequent value of the parameter exceeds the dynamic boundary. 2.The method of claim 1 wherein the parameter is reflected power.
 3. Themethod of claim 1 wherein the parameter is a DC bias.
 4. The method ofclaim 1 wherein the dynamic boundary is computed based upon a filteredvalue of the parameter.
 5. The method of claim 1 wherein the dynamicboundary is computed based upon values of at least two parametersrelated to the power transfer from the RF power generator to the load.6. The method of claim 1 wherein the dynamic boundary comprises an upperlimit and a lower limit.
 7. The method of claim 1 wherein the dynamicboundary is computed based upon a user-defined offset from theparameter.
 8. The method of claim 1, further comprising the step ofextinguishing the arc.
 9. The method of claim 8 wherein the step ofextinguishing the arc comprises interrupting power transfer to the load.10. The method of claim 8 wherein the step of extinguishing the arccomprises reducing power transfer to the load.
 11. The method of claim 8wherein the step of extinguishing the arc occurs during tuning of powertransfer from the RF power generator to the load.
 12. The method ofclaim 8, further comprising the step of holding the dynamic boundaryconstant during the step of extinguishing the arc.
 13. A radio frequencypower delivery system, comprising: a) an RF power generator, wherein theRF power generator provides a measured value of a parameter related topower transfer from the RF power generator to a load; b) arc detectioncircuitry that computes a dynamic boundary about the value of theparameter; and c) controller logic responsive to the arc detectioncircuitry, wherein the controller logic indicates an occurrence of anarc if a subsequent value of the parameter exceeds the dynamic boundary.14. The system of claim 13 wherein the parameter is reflected power. 15.The system of claim 13 wherein the parameter is a DC bias.
 16. Thesystem of claim 13 wherein the arc detection circuitry computes thedynamic boundary based upon a filtered value of the parameter.
 17. Thesystem of claim 16 wherein the arc detection circuitry comprises a fastfilter and a slow filter, and wherein the arc detection circuitrycomputes the dynamic boundary based upon the parameter as filtered bythe slow filter and the controller logic indicates an occurrence of anarc if a subsequent value of the parameter as filtered by the fastfilter exceeds the dynamic boundary.
 18. The system of claim 13 whereinthe arc detection circuitry computes the dynamic boundary based uponvalues of at least two parameters related to the power transfer from theRF power generator to the load.
 19. The system of claim 13 wherein thedynamic boundary comprises an upper limit and a lower limit.
 20. Thesystem of claim 13 wherein the arc detection circuitry computes thedynamic boundary based upon a user-defined offset from the parameter.21. The system of claim 13, further comprising a switch responsive tothe controller logic that acts to extinguish the arc.
 22. The system ofclaim 21 wherein the switch interrupts power transfer to the load toextinguish the arc.
 23. A plasma processing system, comprising: a) aplasma processing chamber; b) an RF power generator that delivers RFpower to a plasma in the plasma processing chamber; c) arc detectioncircuitry that computes a dynamic boundary about the value of theparameter; and d) controller logic responsive to the arc detectioncircuitry, wherein the controller logic indicates an occurrence of anarc if a subsequent value of the parameter exceeds the dynamic boundary.24. The system of claim 23 wherein the parameter is reflected power. 25.The system of claim 23 wherein the parameter is a DC bias.
 26. Thesystem of claim 23 wherein the arc detection circuitry computes thedynamic boundary based upon a filtered value of the parameter.
 27. Thesystem of claim 26 wherein the arc detection circuitry comprises a fastfilter and a slow filter, and wherein the arc detection circuitrycomputes the dynamic boundary based upon the parameter as filtered bythe slow filter and the controller logic indicates an occurrence of anarc if a subsequent value of the parameter as filtered by the fastfilter exceeds the dynamic boundary.
 28. The system of claim 23 whereinthe arc detection circuitry computes the dynamic boundary based uponvalues of at least two parameters related to the power transfer from theRF power generator to the load.
 29. The system of claim 23 wherein thedynamic boundary comprises an upper limit and a lower limit.
 30. Thesystem of claim 23 wherein the arc detection circuitry computes thedynamic boundary based upon a user-defined offset from the parameter.31. The system of claim 23, further comprising a switch responsive tothe controller logic that acts to extinguish the arc.
 32. The system ofclaim 31 wherein the switch interrupts power transfer to the load toextinguish the arc.
 33. The system of claim 23, further comprising animpedance matching network disposed between the RF power generator andthe plasma processing chamber.
 34. The system of claim 31, furthercomprising an impedance matching network disposed between the RF powergenerator and the plasma processing chamber, and wherein the switch actsto extinguish the arc during tuning of the impedance matching network.